Exhaust gas emitted from an internal combustion (IC) engine, is a heterogeneous mixture that may contain gaseous emissions such as carbon monoxide (CO), unburned hydrocarbons (HC) and oxides of nitrogen (NOx) as well as condensed phase materials (liquids and solids) that constitute particulate matter. Catalyst compositions typically disposed on catalyst supports or substrates are provided in an IC engine exhaust system to convert certain, or all of these exhaust constituents into non-regulated exhaust gas components.
In some cases, selective catalytic reductant (SCR) devices are provided to reduce the amount of NOx in the exhaust. The SCR devices make use of ammonia (NH3) to reduce the NOx. For example, when the proper amount of ammonia is available at the SCR device under the proper conditions, the ammonia reacts with the NOx in the presence of an SCR catalyst to reduce the NOx emissions. However, if the reduction reaction rate is too slow, or if there is excess ammonia in the exhaust, ammonia can slip out the exhaust pipe. An ammonia slip catalyst device (ASC) is provided to prevent the slip of ammonia through the exhaust pipe to the atmosphere. Accordingly, it is desirable to diagnose the ASC to ensure operability and to prevent the slip of ammonia to the atmosphere.